Selective Recognition and Reversible “Turn-Off” Fluorescence Sensing of Acetate (CH 3 COO − ) Anion at Ppb Level Using a Simple Quinizarin Fluorescent Dye

: A simple and cost-effective optical sensing system based on quinizarin ﬂuorescent dye ( QZ ) for the selective and reversible sensing of CH 3 COO − anions is reported. The anion binding afﬁnity of QZ towards different anions was monitored using electronic absorption and ﬂuorescence emission titration studies in DMSO. The UV-visible absorption spectrum of QZ showed a decrease in the intensity of the characteristic absorption peaks at λ = 280, 323, and 475 nm, while a new peak appeared at λ = 586 nm after the addition of CH 3 COO − anions. Similarly, the initial strong emission intensity of QZ was attenuated following titration with CH 3 COO − anions. Notably, similar titration using other anions, such as F − , Cl − , I − , NO 3 − , NO 2 − , and H 2 PO 4, caused no observable changes in both absorption and emission spectra. The selective sensing of CH 3 COO − anions was also reﬂected by a sharp visual color change from bright green to faint green under room light. Further, the binding was found to be reversible, and this makes QZ a potential optical and colorimetric sensor for selective, reversible, and ppb-level detection of CH 3 COO − anions in a DMSO medium.


Introduction
The recognition and sensing of anions have gained immense research interest in recent years because of their vital role in various biological, environmental, and industrial processes [1].For instance, fluoride (F − ) ions are important for maintaining dental health and treating osteoporosis [2,3], sulphite (SO 3  2− ) ions are used in the preservation industry [4], and CH 3 COO − ions play a crucial role in various enzymatic and antibody functions [5].Superoxide (O 2 − ) ions, which are reactive oxygen species (ROS), exhibit distinctive activity in cell growth and metabolism [6,7].Phosphate (PO 4 3− ) and halide (X − ) anions are involved in the activity of enzymes, synthesis of protein, hormone transport, and DNA regulation [8][9][10][11].Cyanide (CN − ) ions are frequently employed in metallurgy, electroplating, gold mining, resins, herbicides, and the manufacture of various organic chemicals [12].Although anions are ubiquitous species and play numerous indispensable roles, overexposure to anions can cause severe disorders and be harmful to the environment [13].Therefore, the qualitative and quantitative detection of anions is of paramount interest.
Among various anions, the CH 3 COO − anion in particular has drawn special research focus because of its unique role in biochemical, environmental, and medicinal sciences.It has been a common practice to use the rate of CH 3 COO − generation and oxidation as a gauge for the decomposition of organic materials in marine sediments [14] and the trans-metalation of tetrapyrroles [15,16].The regulation of DNA, hormone transfer, protein synthesis, and enzyme activities are influenced by the recognition of CH 3 COO − ions [8][9][10][11].Considering the vitality of CH 3 COO − ions in both biological and environmental aspects, developing suitable sensor systems for the selective, sensitive detection and quantification of CH 3 COO − is an important topic of research.Designing colorimetric and fluorescent anion sensors is of great importance in the current scenario due to their simple procedure, low cost, high sensitivity, and low detection limit [17,18].In the recent past, several small-molecule-based chemosensors have been developed and successfully employed for selective anion recognition and sensing [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36].For the detection of anions, the most prevalent chemosensors are polar -NH protons of urea [37] or thiourea [38], amide [39], pyrrole [40], and imidazolium [41,42] moieties.Among the different non-covalent interactions studied, hydrogen-bonding interactions are particularly beneficial and successful in the interaction of anions with a sensor.Coordinative interactions have also been extensively reported by sensors using metal ions.Sensors that use hydroxyl (-OH) groups as hydrogen bond donors have been explored to a lesser extent, although they can form effective hydrogen bonds with anions or induce deprotonation on anion coordination [43,44].
Anthraquinones are an important class of organic small-molecule fluorescent dyes that have been used extensively for the selective detection of anions, cations, and toxic small molecules [45,46].Anthraquinone derivatives are substances of substantial chemical and biological interest.For many years, the chromophoric system of anthraquinones has been widely utilized to make textile dyes [47].In addition, these compounds play a significant role as a class of biologically relevant and pharmaceutically active chromophores as well as an analytical tool for metal sensing [46,48,49].Quinizarin (QZ, 1,4-dihydroxyanthraquinone) is one type of fluorescent anthraquinone with two hydroxyl groups (See Figure 1 for structure).In this report, we were interested in examining the use of structurally simple QZ as a selective and reversible fluorescent sensor for anions.We anticipated that the presence of two -OH groups could act as receptor sites for anion recognition, either through hydrogen-bonding interactions or deprotonation of phenolic-OH protons following the binding of anions that subsequently perturbs the optical properties of the QZ sensor.Indeed, electronic absorption and fluorescence titration studies showed that QZ can selectively bind with CH non-covalent interactions studied, hydrogen-bonding interaction ficial and successful in the interaction of anions with a sensor.C have also been extensively reported by sensors using metal ion droxyl (-OH) groups as hydrogen bond donors have been expl although they can form effective hydrogen bonds with anions o on anion coordination [43,44].
Anthraquinones are an important class of organic small-mo that have been used extensively for the selective detection of an small molecules [45,46].Anthraquinone derivatives are substanc cal and biological interest.For many years, the chromophoric sy has been widely utilized to make textile dyes [47].In addition, t significant role as a class of biologically relevant and pharmace phores as well as an analytical tool for metal sensing [46,4 1,4-dihydroxy-anthraquinone) is one type of fluorescent anthra droxyl groups (See Figure 1 for structure).In this report, we were the use of structurally simple QZ as a selective and reversible flu ions.We anticipated that the presence of two -OH groups could anion recognition, either through hydrogen-bonding interactio phenolic-OH protons following the binding of anions that subseq tical properties of the QZ sensor.Indeed, electronic absorption an studies showed that QZ can selectively bind with CH3COO − an quenching-based sensing mechanism.The initial fluorescence em was quenched following the addition of CH3COO − anions.Notab titration studies using other anions, such as F − , Cl − , I − , NO3 − , NO2 negligible binding affinity with QZ.Therefore, QZ dye could be a for the selective and reversible sensing of CH3COO − anions.

Materials
The reagents and chemicals were obtained from various c used as received, without further purification.For the titration

Materials
The reagents and chemicals were obtained from various commercial sources and used as received, without further purification.For the titration experiments, all anions were added either as tetrabutyl ammonium or sodium salts.The sensor Quinizarin is commercially available and was purchased from Sigma-Aldrich (Bangalore, India) and used as received.The solvents used for spectroscopic studies were purchased from various suppliers and were HPLC grade.

Instrumentations
The UV-visible absorption spectra were measured in quartz cuvettes using Thermo Scientific evolution 201 (Waltham, MA, USA) spectrometers.Baseline corrections were applied to all measurements.The fluorescence emission spectra were recorded using the Perkin Elmer-6500 (Waltham, MA, USA) Fluorimeter.All measurements were carried out at 298 K, and spectral data were processed using OriginPro 8.5 (Waltham, MA, USA).

Fluorescence Sensing Studies
For the titration studies, stock solutions of the QZ sensor (1 mM) and different anions (1 mM) as sodium salts, specifically F − , NO 3 − , and NO 2 − , or tetrabutyl ammonium salts, specifically CH 3 COO − , Cl − , I − , and H 2 PO 4 − , were freshly prepared in DMSO.In a cuvette, 200 µL of 1 mM DMSO solution of QZ was taken and diluted with 1800 µL of fresh DMSO solvent.CH 3 COO − , F − , Cl − , I -, NO 3 − , NO 2 − , and H 2 PO 4 − salts were added to this solution separately in increments of 20 µL to obtain concentrations up to 90.9 µM.The fluorescence emission intensity of QZ was monitored before and after the addition of different anions at various concentrations.For each fluorescence titration experiment, the sample was excited at λ = 437 nm and the fluorescence quenching efficiency was calculated using Equation (1): where I 0 is the initial emission intensity of QZ in DMSO and I is the intensity after the addition of the anions.The Stern-Volmer quenching constant (K SV ) was obtained by fitting the data using the following Stern-Volmer Equation ( 2): K SV was calculated from the slope of the linear curve of the relative changes in emission intensity (I 0 /I) vs. the concentration [Q] of CH 3 COO − ions.

Anion Sensing Studies Using UV-Vis Titrations
At first, the anion-binding properties of QZ were investigated by UV-Vis absorption studies at 25 • C. Titrations were carried out using 1 mM solutions of QZ in the DMSO solution.The absorption spectra of QZ were recorded before and after the incremental addition (0.0 to 90.9 µM) of the standard solution (1 mM) of different anions: − , NO 2 − , and NO 3 − .As shown in Figure 2A, the DMSO solution of QZ (10 µM) exhibited typical anthraquinone characteristic absorption bands: a high-energy band at λ = 280 nm (ε = 1.2 × 10 4 M −1 cm −1 ), a shoulder band at λ = 323 nm (ε = 3.5 × 10 3 M −1 cm −1 ), and a low-energy broad peak centered at λ = 475 nm (ε = 9.6 × 10 3 M −1 cm −1 ).After the addition of 10 equiv. of CH 3 COO − ions, the electronic absorption intensity of the bands at λ = 280 nm and 475 nm decreased significantly, which was accompanied by the appearance of a longer wavelength with a new absorption band at λ = 586 nm corresponding to the complex formation between QZ and CH 3 COO − anions (Figure 2A).Notably, the appearance of a clear isosbestic point at λ = 525 nm was consistent with an equilibrium existence of complexed and free QZ with CH 3 COO − anions in the DMSO solution (Figure 2C).The significant changes in absorption intensity of QZ after the addition of CH 3 COO − anions were also reflected by a sharp visual color change from bright green to faint green under room light before and after the mixing of QZ with CH 3 COO − anions (Figure 2A).Interestingly, similar absorption titration studies using 10 equiv. of other competing anions, such as F − , Cl − , I − , NO 3 − , NO 2 − , and H 2 PO 4 − , showed only little changes in the absorption spectra of QZ, and these results demonstrate that QZ can discriminatively and selectively bind with CH 3 COO − anions (Figure 2B).From the electronic absorption titration profile, a perfect linear plot of 1/∆A vs. 1/[CH 3 COO − ] was obtained using the Benesi-Hildebrand equation and from the slope of this linear plot; moreover, the association constant K a for CH 3 COO − ion binding was determined to be 7.63 × 10 3 M −1 , which is comparable to already reported different optical sensors for CH 3 COO − ions (Figure 2D) [26,35,50].The high value of K a indicates the strong binding affinity of QZ for CH 3 COO − anions in the DMSO solution.
istry 2022, 4, FOR PEER REVIEW ions, such as F − , Cl − , I − , NO3 − , NO2 − , and H2PO4 − , showed only little changes in the absor tion spectra of QZ, and these results demonstrate that QZ can discriminatively and lectively bind with CH3COO − anions (Figure 2B).From the electronic absorption titrati profile, a perfect linear plot of 1/ΔA vs. 1/[CH3COO − ] was obtained using the Bene Hildebrand equation and from the slope of this linear plot; moreover, the associati constant Ka for CH3COO − ion binding was determined to be 7.63 × 10 3 M −1 , which comparable to already reported different optical sensors for CH3COO − ions (Figu 2D).[26,35,50] The high value of Ka indicates the strong binding affinity of QZ f CH3COO − anions in the DMSO solution.

Anion Sensing Using Fluorescence Titrations Studies
The anion-binding propensity of QZ was further monitored using fluorescence tration studies in the DMSO solution.The QZ sensor was highly emissive due to t presence of emissive anthraquinone moiety and exhibited a broad emission band ma mum at λ = 562 nm, which was quenched drastically after the addition of increasi concentrations of CH3COO − anions (0.0 to 10 equiv.)(Figure 3C).The observed attenu tion in emission intensity after the anion addition was presumably due to the depro nation of the phenolic-OH group of QZ following CH3COO − interactions.This bindi facilitates the photoinduced electron transfer (PET) from O to π* orbitals of anthraq none moiety that results in the subsequent quenching of the fluorescence emission tensity (Scheme 1).

Anion Sensing Using Fluorescence Titrations Studies
The anion-binding propensity of QZ was further monitored using fluorescence titration studies in the DMSO solution.The QZ sensor was highly emissive due to the presence of emissive anthraquinone moiety and exhibited a broad emission band maximum at λ = 562 nm, which was quenched drastically after the addition of increasing concentrations of CH3COO − anions (0.0 to 10 equiv.)(Figure 3C).The observed attenuation in emission intensity after the anion addition was presumably due to the deprotonation of the phenolic-OH group of QZ following CH 3 COO − interactions.This binding facilitates the photoinduced electron transfer (PET) from O to π* orbitals of anthraquinone moiety that results in the subsequent quenching of the fluorescence emission intensity (Scheme 1).The proposed sensing mechanism of anion-induced deprotonation of phenolic-OH groups of QZ following CH3COO − anion addition was further validated by pH-dependent fluorescence titration studies in the DMSO solution.The fluorescence emission intensity of QZ was checked under different pH levels from 3.5 to 11.5.As given in Figure S1, the obtained results revealed that QZ is strongly emissive under acidic pH less than 5, and the fluorescence emission intensity was found to decrease under basic pH up to 7. With further increases in pH from 10 to 11.5, QZ was almost poorly emissive, presumably due to the deprotonation of phenolic-OH groups under basic pH, which resulted in significant fluorescence quenching of QZ due to the proposed PET transition from O to π* orbitals of anthraquinone.We further performed computational calcula-  The proposed sensing mechanism of anion-induced deprotonation of phenolic-OH groups of QZ following CH 3 COO − anion addition was further validated by pH-dependent fluorescence titration studies in the DMSO solution.The fluorescence emission intensity of QZ was checked under different pH levels from 3.5 to 11.5.As given in Figure S1, the obtained results revealed that QZ is strongly emissive under acidic pH less than 5, and the fluorescence emission intensity was found to decrease under basic pH up to 7. With further increases in pH from 10 to 11.5, QZ was almost poorly emissive, presumably due to the deprotonation of phenolic-OH groups under basic pH, which resulted in significant fluorescence quenching of QZ due to the proposed PET transition from O to π* orbitals of anthraquinone.We further performed computational calculations to probe the cyclic stability of QZ and support the proposed PET transition.The energy-minimized structure of QZ verified that the proposed hydrogen-bonded cyclic structure is feasible and stable (see Figure S2).Moreover, the highest-occupied molecular orbital (HOMO) of deprotonated QZ 2− was higher in energy than the lowest-unoccupied molecular orbital (LUMO) of the QZ sensor, which confirms that the existence of the PET transition from O to π* orbitals of anthraquinone causes the observed fluorescence quenching.A perfectly linear Stern-Volmer plot was obtained from the fluorescence titration data, and the calculated Stern-Volmer quenching constant was found to be K SV = 2.3 × 10 3 M −1 .The high value of K SV , again, confirms the strong binding affinity of QZ for CH 3 COO − anions (Figure 3D).
To establish the sensing selectivity of QZ towards CH 3 COO − ions, we performed similar fluorescence titration studies using other competing anions, such as F − , Cl − , I − , H 2 PO 4 − , NO 2 − , and NO 3 − .As depicted in Figure 3A, only CH 3 COO − anions elicited significant fluorescence quenching with no other spectral changes seen, whereas F − , Cl − , I − , NO 2 − , and NO 3 − anions showed almost negligible fluorescence quenching effects.Thus, QZ can be a selective "turn-off" fluorescent sensor for CH 3 COO − anions.To further corroborate the high selectivity, we carried out competitive fluorescence sensing studies on other anions under the same experimental conditions.As shown in Figure 3B, the initial fluorescence emission intensity (red bar) of QZ did not change (green bar) significantly after the addition of different anions, such as F − , Cl − , I − , H 2 PO 4 − , NO 2 − , and NO 3 − .However, the subsequent mixing of CH 3 COO − anions exhibited a dramatic fluorescence quenching (blue bar).These results further validated the high selectivity of QZ for CH 3 COO − anion detection.It is worth mentioning that in the presence of H 2 PO 4 − , CH 3 COO − anions showed a moderate fluorescence quenching effect, which may be due to the possible interference from H 2 PO 4 − anions because of their similar basicity and charge density with CH 3 COO − anions [50].
Gratifyingly, the observed selective sensing of CH 3 COO − anions was also clearly visible to the naked eye.As shown in Figure 4A, the initial bright-orange color of QZ changed to colorless after adding CH 3 COO − anions under UV light irradiation, and a bright green solution changed to faint green under room light.However, no visible color changes were observed for QZ in presence of other anions; thus, the fluorescent dye QZ can be used as a colorimetric probe for CH 3 COO − ion detection.Moreover, we fabricated a filter paper-based colorimetric sensor for the detection of CH 3 COO -ions.As depicted in Figure 4B, the initial green emission of drop-casted QZ on filter paper changed to nonfluorescent after mixing a drop of the CH 3 COO − solution; thus, QZ can be realized as a suitable sensor for the detection of CH 3 COO − anions both in solution and solid phases.
Chemistry 2022, 4, FOR PEER REVIEW 6 tions to probe the cyclic stability of QZ and support the proposed PET transition.The energy-minimized structure of QZ verified that the proposed hydrogen-bonded cyclic structure is feasible and stable (see Figure S2).Moreover, the highest-occupied molecular orbital (HOMO) of deprotonated QZ 2− was higher in energy than the lowest-unoccupied molecular orbital (LUMO) of the QZ sensor, which confirms that the existence of the PET transition from O to π* orbitals of anthraquinone causes the observed fluorescence quenching.A perfectly linear Stern-Volmer plot was obtained from the fluorescence titration data, and the calculated Stern-Volmer quenching constant was found to be KSV = 2.3 × 10 3 M −1 .The high value of KSV, again, confirms the strong binding affinity of QZ for CH3COO − anions (Figure 3D).
To establish the sensing selectivity of QZ towards CH3COO − ions, we performed similar fluorescence titration studies using other competing anions, such as F − , Cl − , I − , H2PO4 − , NO2 − , and NO3 − .As depicted in Figure 3A, only CH3COO − anions elicited significant fluorescence quenching with no other spectral changes seen, whereas F − , Cl − , I − , NO2 − , and NO3 − anions showed almost negligible fluorescence quenching effects.Thus, QZ can be a selective "turn-off" fluorescent sensor for CH3COO − anions.To further corroborate the high selectivity, we carried out competitive fluorescence sensing studies on other anions under the same experimental conditions.As shown in Figure 3B, the initial fluorescence emission intensity (red bar) of QZ did not change (green bar) significantly after the addition of different anions, such as F − , Cl − , I − , H2PO4 − , NO2 − , and NO3 − .However, the subsequent mixing of CH3COO − anions exhibited a dramatic fluorescence quenching (blue bar).These results further validated the high selectivity of QZ for CH3COO − anion detection.It is worth mentioning that in the presence of H2PO4 − , CH3COO − anions showed a moderate fluorescence quenching effect, which may be due to the possible interference from H2PO4 − anions because of their similar basicity and charge density with CH3COO − anions [50].
Gratifyingly, the observed selective sensing of CH3COO − anions was also clearly visible to the naked eye.As shown in Figure 4A, the initial bright-orange color of QZ changed to colorless after adding CH3COO − anions under UV light irradiation, and a bright green solution changed to faint green under room light.However, no visible color changes were observed for QZ in presence of other anions; thus, the fluorescent dye QZ can be used as a colorimetric probe for CH3COO − ion detection.Moreover, we fabricated a filter paper-based colorimetric sensor for the detection of CH3COO -ions.As depicted in Figure 4B, the initial green emission of drop-casted QZ on filter paper changed to non-fluorescent after mixing a drop of the CH3COO − solution; thus, QZ can be realized as a suitable sensor for the detection of CH3COO − anions both in solution and solid phases.To meet the practical uses, the sensor must be reversible and fluorescence sensing properties should be regenerable.To verify this, we carried out a fluorescence titration experiment employing the polar solvent H 2 O as a potential competing solvent to establish the reversibility of QZ for CH 3 COO − sensing.As seen in Figure 5A, the addition of H 2 O can effectively protonate O -to form back the original OH groups, thereby leading to the recovery of emission intensity of unbound QZ dye.The sensitivity of QZ was additionally determined from the plot of changes in emission intensity as a function of CH 3 COO − anions at minute concentrations (Figure 5B).From the plot, the limit of detection (LoD) was determined to be 38 ppb, which is quite low for the detection of CH 3 COO − ions prevalent in many chemical and biological systems.
Chemistry 2022, 4, FOR PEER REVIEW 7 To meet the practical uses, the sensor must be reversible and fluorescence sensing properties should be regenerable.To verify this, we carried out a fluorescence titration experiment employing the polar solvent H2O as a potential competing solvent to establish the reversibility of QZ for CH3COO − sensing.As seen in Figure 5A, the addition of H2O can effectively protonate O -to form back the original OH groups, thereby leading to the recovery of emission intensity of unbound QZ dye.The sensitivity of QZ was additionally determined from the plot of changes in emission intensity as a function of CH3COO − anions at minute concentrations (Figure 5B).From the plot, the limit of detection (LoD) was determined to be 38 ppb, which is quite low for the detection of CH3COO − ions prevalent in many chemical and biological systems.The sensitivity plot indicates that the limit of detection is at the ppb level.

Conclusions
In conclusion, we have presented a simple quinizarin (QZ)-based small-molecule optical sensor for the selective and reversible sensing CH3COO − anions in DMSO.Electronic absorption and fluorescence titration studies demonstrated that the QZ sensor could selectively recognize CH3COO − in presence of various other competing anions (F − , Cl − , I − , NO3 − , NO2 − , and H2PO4 − ).The initial emission intensity of QZ was remarkably quenched upon the addition of CH3COO − ions, which was attributed to the deprotonation of phenolic -OH groups of QZ following CH3COO − anion addition.Moreover, the observed selective binding of CH3COO − anions was also reflected by sharp visual color changes both in solution and solid phases.In summary, the CH3COO − anion sensing properties of the fluorescent dye QZ are comparable to several reported small-molecule-based fluorescent sensors (see Table S1); hence, QZ can be a possible optical and colorimetric sensor for the detection and quantification of CH3COO − anions.Further work is in progress to functionalize QZ to make it a practically feasible water-soluble sensor for its application in the chemosensing of biologically relevant and environmentally concerned analytes.
3 COO − anions via a fluorescence quenching-based sensing mechanism.The initial fluorescence emission of QZ in DMSO was quenched following the addition of CH 3 COO − anions.Notably, similar fluorescence titration studies using other anions, such as F − , Cl − , I − , NO 3 − , NO 2 − , and H 2 PO 4 − , exhibited negligible binding affinity with QZ.Therefore, QZ dye could be a potential optical sensor for the selective and reversible sensing of CH 3 COO − anions.

Figure 2 .
Figure 2. (A) The UV-visible absorption spectrum of QZ before and after the addition of 10 equ of CH3COO -anions (Inset: corresponding visual color changes).(B) The observed changes in sorption intensity of QZ after the addition of 10 equiv. of different anions.(C) Relative changes UV-visible absorption intensity after increasing CH3COO -anion concentrations.(D) Correspon ing Benesi-Hildebrand plot.

Figure 2 .
Figure 2. (A) The UV-visible absorption spectrum of QZ before and after the addition of 10 equiv. of CH 3 COO -anions (Inset: corresponding visual color changes).(B) The observed changes in absorption intensity of QZ after the addition of 10 equiv. of different anions.(C) Relative changes in UV-visible absorption intensity after increasing CH 3 COO -anion concentrations.(D) Corresponding Benesi-Hildebrand plot.

Figure 3 .Scheme 1 .
Figure 3. (A) The changes in fluorescence emission intensity of QZ (10 μM) after the addition of different anions (90.9 μM).(B) The competitive binding ability of QZ towards CH3COO − ions in the co-existence of other anions.(C) The relative changes in emission intensity of QZ after increasing the concentration of CH3COO − .(D) Corresponding Stern-Volmer plot.

Figure 3 .
Figure 3. (A) The changes in fluorescence emission intensity of QZ (10 µM) after the addition of different anions (90.9 µM).(B) The competitive binding ability of QZ towards CH 3 COO − ions in the co-existence of other anions.(C) The relative changes in emission intensity of QZ after increasing the concentration of CH 3 COO − .(D) Corresponding Stern-Volmer plot.

Figure 3 .Scheme 1 .Scheme 1 .
Figure 3. (A) The changes in fluorescence emission intensity of QZ (10 μM) after the addition of different anions (90.9 μM).(B) The competitive binding ability of QZ towards CH3COO − ions in the co-existence of other anions.(C) The relative changes in emission intensity of QZ after increasing the concentration of CH3COO − .(D) Corresponding Stern-Volmer plot.

Figure 4 .
Figure 4. (A) The observed visual color changes for QZ in DMSO solution after the addition of different anions.(B) The color changes observed for QZ coated on filter paper before and after the mixing of CH3COO − anions.

Figure 4 .
Figure 4. (A) The observed visual color changes for QZ in DMSO solution after the addition of different anions.(B) The color changes observed for QZ coated on filter paper before and after the mixing of CH 3 COO − anions.

Figure 5 .
Figure 5. (A) Regeneration of fluorescence emission of QZ following titration with H2O at 25 °C.(B)The sensitivity plot indicates that the limit of detection is at the ppb level.